Each chemistry set has a number of charges, which in-game represent its stock of potion ingredients, and are consumed when the set is used to make a potion. Each potion type in the game has a specific ingredient cost (see below).

Like writing scrolls, making a potion requires a "blank" item—an empty bottle.

The beatitude of the resulting potion is the average of the beatitudes of the set and the bottle, the same way as with magic marker.

You can make potions you have not discovered yet, with a small chance of success.

Unlike writing scrolls, making potions requires knowledge of chemistry. Potions made by the player are "selfmade" and have a luck-dependent chance of being defective.

Note that a chemistry set is made up of fragile glassware, which can shatter easily if thrown or kicked.

Prerequisites

Spellbook of chemistry

Before you can use a chemistry set with certainty, you need to be familiar with chemistry. This can be learned by successfully reading a spellbook of chemistry. As long as you have the chemistry "spell" in your repertoire, you can use chemistry sets. Note that this is not a functional spell, and attempting to cast it will only produce a funny message ("You call upon your chemical knowledge. Nothing happens.").

If you try to use a chemistry set without knowing the "chemistry" spell, you may get the message "Huh? You don't understand anything about such stuff!" and one charge on the chemistry set will be used up.

Trying to use a chemistry set without knowing chemistry will always fail in NHTNG. In Slash'EM Extended you have a 33% chance of being able to make a
potion even if you don't know the spell; otherwise, you will get the same message and lose a charge.

Slash'EM Extended objects.c line 2108

Bottle

Before you can make a new potion, you must have an empty bottle to contain it. The first thing that happens when you use a chemistry set is that you will be prompted to select a container from your inventory.

Empty bottles can be found as randomly generated tools, but a much more common source of bottles is quaffing potions, which will sometimes leave behind the empty container. ("You are left with the empty bottle.") If the potion was taken from a stack, so a new inventory slot is needed for the bottle, but there is no room in your pack, the bottle will drop to the floor. Bottles acquired in this way are naturally uncursed, regardless of the BUC status of the original potion.

Also, if you quaff a milky potion but no ghost appears, presumably due to extinction, you get to keep the empty bottle.

Failure

If the chemistry set doesn't have enough charges to make your desired potion, the bottle will explode ("You seem to have made a mistake!") dealing 10 damage, or 25 if the chemistry set was cursed. Getting killed by such an explosion will give the report that you were killed by an "alchemic blast".

If you attempt to make a potion you have not yet discovered, you have a luck-weighted chance of succeeding; otherwise, the bottle will explode.

Selfmade potions

Potions made using chemistry sets are flagged "selfmade" (compare homemade tins) and can be defective. When you quaff one, there is a small chance that it will be "bad" (due to an error in the preparation) and possibly have harmful effects, poisoning you for up to 20 hit points OR making you nauseated. A death resulting from poisoning from a selfmade potion will be attributed to "bad chemical knowledge"; however, you will not lose any health if you have poison resistance.

The probability that a selfmade potion will be "bad" and the probability that quaffing it will have a negative effect are each luck-weighted, so the higher your luck, the less likely you are to suffer harmful effects from quaffing your own potions.

Charging

Like most tools, a chemistry set can be recharged an indefinite number of times. An uncursed scroll of charging adds 1-5 charges. A blessed scroll adds 1-10 charges. A cursed scroll sets the number of charges to zero, unless the chemistry set is blessed.

The maximum number of charges a chemistry set can have is 117. This may have something to do with the number of elements on the current periodic table (118).